Forest evapotranspiration (ET) has long been recognized to be a major component in the mass water balance of subalpine watersheds in the central Rocky Mountains. In recent years, research has focused on the processes involved in ET and in the way those processes are influenced by forest cultural activities. Only a few studies have addressed how species differ in annual transpirational or ET losses. Yet species differences could have significant long-term hydrologic effects. This paper synthesizes the results of a series of studies conducted on transpiration of the four major tree species in subalpine forests of the central Rocky Mountains - Englemann spruce, subalpine fir, lodgepole pine, and aspen. Research was conducted at the Fraser Experimental Forest near Fraser, Colo. , an area typical of such forests. A model for estimating transpiration is described, followed by an assessment of species differences in annual transpiration.